Plesiosaurs are much like the sauropod dinosaurs in that they have an extremely long neck. They lived in the Mesozoic seas alongside ichthyosaurs and pliosaur, and went extinct at the end of the Mesozoic, 65 million years ago.
Plesiosaurs are very closely related to pliosaurs. Like pliosaurs, they had four very large fins that were used for swimming. Plesiosaurs also had very long necks and tails.
The first plesiosaur to be named and described was Plesiosaurus back in 1821. It came from the south coast of England, near Lyme Regis in Dorset. It was found by Mary Anning, one of the world’s first professional fossil collectors.